1. Field of the Invention
The invention relates to a power tool, in particular an electrically operable hand-held power tool.
2. Description of the Prior Art
Power tools that provide at least a rotary drive of insert bits usually have overload clutches that limit a torque acting on the insert bit to a particular preselected limit torque. Particularly in hand-held power tools with powerful rotary drive units such as drills, impact drills, or rotary/combi hammers, the preselected limit torque and therefore the overload clutch play an important role in accident prevention. Usually, the preselected limit torque is a compromise between the lowest possible torque load on the insert bit, the power tool, and/or a user and the highest possible working torque for using large tool diameters of the insert bit.
Overload clutches or clutch devices with adjustable limit torques are provided so as to meet the requirements of different work situations. DE 43 04 899 A1 has disclosed a rotary hammer as an example of a hand-held power tool in which at least two separate clutches are provided in the transmission device between a drive source and a rotary drivable tool holder. In this case, the clutches have different limit torques and can be brought into engagement by the user independently of each other so that different limit torques can be in effect during use of the rotary hammer.
In addition, various embodiments of two-stage clutch devices are already known. DE 38 32 302 C1 has disclosed a clutch device with two overload clutches that can be coupled and are arranged in series on a shaft. The overload clutches in this case are selectively connected to each other and/or fastened to a housing by means of a switchable, axially movable pin that is situated in a radially outer region of the overload clutches so that different limit torques are in effect in relation to the shaft.
DE 101 30 520 A1 has also disclosed a clutch device with an overload clutch for hand-held power tools in which the limit torque of the overload clutch can be set by means of a control element. For this purpose, the overload clutch has radially situated spring elements, which are set to a certain prestressing force in order to produce an overload torque and whose prestressing force can be changed by means of a control element.
Usually, even with the use of very powerful power tools, a very high working torque is only required in relatively few specific applications. In the most frequent applications by far, a very good work result can be achieved with a relatively low working torque. An automatic resetting to a lower working torque thus advantageously increases user-friendliness. WO 2004/024398 A1 has disclosed a rotary hammer with a two-stage clutch device in which an automatic resetting of the clutch device to the lowest possible limit torque is triggered as soon as the rotary hammer is disconnected from the line voltage.